It is well known that all electrolytic metal refining or recovery processes are limited, insofar as the applicable current density is concerned, by the rate with which metal ions are diffused from the electrolyte into the liquid film layer adhering to the cathode surface. The higher the metal deposition rate on the cathode, and thus the higher the depletion rate of metal ions from the cathode film, the more this limitation affects the current efficiency, and the smoothness, crystal structure and density of the deposit. Specifically, when the rate of metal ion removal from the cathode film for deposit onto cathode surface exceeds the diffusion rate of metal ions from the electrolyte into the cathode film for replenishment, a considerable portion of the current is made available for hydrogen deposition rather than metal deposition. Under these conditions crystal growth does not occur parallel with the cathode surface, the resulting metal deposits are of poor quality in that they usually are powdery, rough-textured, poorly adhering coatings of insufficient thickness. Also, more frequent shut-downs for cleaning of the cell are required to prevent short-circuiting caused by bridging of the electrodes by metallic deposits, which have either flaked off from the cathode into the electrolyte or have grown out of the cathode surface as so called "dentrites", i.e. irregular tree-like formations.
When using soluble metal anodes high current density electrodissolution creates a somewhat similar problem inasmuch as the metal is dissolved from the anode at a greater rate than the rate of diffusion of the metal into the main body of the electrolyte. As a result, the anode film layer becomes enriched in metal salts to such an extent that it becomes highly viscous and also depleted in solvent anions, the resistance is greatly increased, the current flow is impeded, and the desired smooth, uniform dissolution is affected.
It is apparent from the above that there is a maximum or "limiting" current density that can be used in any particular electrolytic system for deposits of metal of acceptable quality, especially if the aim is to build up a heavy deposit, such as is the case in most commercial electrowinning or refining processes. Since the current density that can be employed is directly related to the surface area of the electrodes and therefore the size and capital cost of the entire electrolytic cell, it follows that any improvement, which serves to increase the "limiting" current density without adding significant further costs would be highly desirable.
Generally, it has been recognized by those familiar with the art that the aforementioned diffusion rate decreases with increasing electrode film thickness and therefore, a reduction of this film thickness is one of the best approaches for solving the problem. Agitation, i.e. a rapid movement of the electrodes or the electrolyte relative to each other is most helpful in this respect. For the agitation to be meaningful it should act parallel to the electrode surface.
Various methods of agitation have been suggested and used with limited success including mechanical movement of the electrodes and direct movement of the electrolyte. Of the former, the most common method is mechanical reciprocation of the electrodes, however, vertical or horizontal electrode oscillation or rotation of a circular electrode are other possible methods of agitation by electrode movement. Mechanical movement of the electrodes has obvious physical limitations. Since the electrode and bus-bar assembly are massive and cumbersome, it is not practical to accelerate them to high velocities and then decelerate to a stop in order to achieve a reciprocal motion. In practice, the maximum velocity that can be achieved during such reciprocation is about 15 ft/min, giving an average effective overall velocity of about 5 ft/min.
Electrolyte solution movement can be achieved by circulation of air through the electrolyte or by circulation of the solution through pumping. The latter is the most common method of moving the electrolyte past the electrodes. Its main drawback is that while at the pumping discharge the agitation can be very efficient, as the energy is being dispersed, the direction of the solution flow cannot be controlled over a larger surface, back pressure impediments to the flow occur, eddy currents are generated, and the desired uniformity of solution agitation cannot be maintained. In general, the solution movement that can be achieved through recirculation by pumping in commercial processes is quite low, typically in the order of less than 1 ft/min.
The current density that can be used in commercial electrolytic refining and recovery of metals has therefore been limited for practical reason to rather low values. For instance, when the metal is copper, the limiting current density is typically about 25 amps/sq.ft.
U.S. Pat. No. 4,053,377 discloses an electrolytic cell for electrodeposition of copper wherein some of these drawbacks of maintaining a high-velocity, uniform solution flow past the electrodes are overcome and wherein current densities in the range from 60 to 400 amps/sq.ft. are employed in the copper plating. Specifically, the electrolyte is introduced by means of an external centrifugal pump to the cell and passed through a series of baffles having increasing numbers of orifices into a venturi section, then through a narrow channel formed by a single cathode-anode pair. The electrolyte thereafter flows through an enlarged chamber and exits the cell via a conduit, which is connected to the suction inlet of the above-mentioned external pump. The dimensions of the cell are required to provide a uniform rate of movement of electrolyte past the electrode pair of at least 75 ft/min, and preferably of about 150 to 400 ft/min.
From an economical standpoint this cell design is impractical for use in commercial scale operations. One reason for this is that since a major portion of the cell tank is occupied by the baffle plates, the venturi section and the exit chamber, in which no plating takes place, and since the design only provides for one cathode plating surface per cell, the plating capacity per unit area of floor space occupied by the cell is extremely low.
Another reason is that the power requirements needed for recirculation of the electrolyte is excessive. Considering that in a commercial size cell the spacing between the anode and cathode surfaces should be sufficiently wide to permit build-up of a relatively thick deposit on the cathode surface before it is replaced, it follows that large volumes of the electrolyte must be pumped past the electrode surfaces at the required high lineal velocities. Since considerable energy losses are caused by the high velocity recirculation of the electrolyte by way of narrow pipes and with several rapid directional changes, and since additional considerable energy losses are encountered in passing the electrolyte through the series of apertured baffle plates, the use of external pipes and pumping means are highly inefficient in commercial applications of this cell.
It is, therefore, an object of the present invention to provide a novel electrolytic cell, wherein a moderate-velocity uniform, parallel movement of the electrolyte past all electrode surfaces is maintained while minimizing energy losses in moving said electrolyte. In addition to maximizing electrolyte velocity per unit of energy input, another object is to provide a practical, high-capacity cell design, which is economically feasible for commercial high-quality plating applications at high current densities. Other objects of the invention will become apparent from a reading of the specification, drawings and the appended claims.